Artifacts

The art of EEG interpretation lies in being able to distinguish signals generated by brain from signals generated from somewhere else. "Artifact" refers to the anything else, although it should be kept in mind that many "artifacts" are actually quite useful. Some of the more frequently encountered sources of artifactual EEG signals include:

muscle

eyes

electrical equipment, 60 Hz and otherwise

movement

EKG

If you know a handful of ways that muscle artifact is frequently misinterpreted as epileptiform, then you are doing very well!